1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor substrate. In addition, the present invention relates to a semiconductor device manufactured using the semiconductor substrate.
2. Description of the Related Art
In recent years, integrated circuits using an SOI (silicon on insulator) substrate in which a thin single crystal semiconductor layer is formed on an insulating surface, instead of a bulk silicon wafer, have been developed. By utilizing characteristics of a thin single crystal silicon layer formed over an insulating film, transistors formed in the integrated circuit can be electrically separated from each other completely. Further, each transistor can be formed as a fully-depleted transistor, and thus a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low voltage consumption can be realized.
As a method for manufacturing an SOI substrate, a hydrogen ion implantation separation method in which hydrogen ion implantation and separation are combined is known. A summary of a method for manufacturing an SOI substrate by a hydrogen ion implantation separation method is described below. First, by implantation of hydrogen ions into a silicon wafer serving as a separation substrate by an ion implantation method, an ion-implanted layer is formed at a predetermined depth from the surface. Next, the silicon wafer into which hydrogen ions are implanted is bonded to another silicon wafer with a silicon oxide film interposed therebetween. Then, through heat treatment, the ion-implanted layer serves as a cleavage plane and separation into a thin film occurs in the silicon wafer to be separated into which hydrogen ions are implanted. Accordingly, a single crystal silicon film can be formed over the silicon wafer to be separated. The hydrogen ion implantation separation method may also be referred to as a Smart Cut (registered trademark) method.
A method for forming a single crystal silicon film over a base substrate made of glass by using a hydrogen ion implantation separation method as described above has been proposed (for example, see Patent Document 1: Japanese Published Patent Application No. H11-097379 and Patent Document 2: Japanese Published Patent Application No. 2000-294754). In Patent Document 1, a separation plane is mechanically polished to remove a defect layer which is formed by ion implantation or a step of several to several tens of nanometers in the separation plane. In Patent Document 2, planarity of the single crystal semiconductor layer is improved by performing heat treatment such as laser light irradiation on the single crystal semiconductor layer after a separation process.